1. Field of the Invention
The present invention relates to molecular beam epitaxial growth apparatus and method.
2. Description of the Prior Art
A molecular beam epitaxy (MBE) is suitable for growing a thin compound semiconductor film with high controllability. In this method, elements as constituents of the semiconductor film are used as source materials. For example, in order to grow doped aluminum gallium arsenide, aluminum, gallium, arsenic, and dopants are stored in Knudsen cells, respectively. These sources are heated to generate molecular beams independently. The Knudsen cells are arranged such that the central portion of their beam flux distribution intersects the substrate heated by a heating device. An ion gauge for measuring the respective molecular beam intensities is movably arranged inside the growth apparatus to control the intensities.
A molecular beam epitaxial growth apparatus is an ultra high vacuum apparatus. A frequency for opening the growth chamber to the air must be reduced to obtain a thin semiconductor film of a high purity and to obtain high efficiency of apparatus operation. In order to reduce contamination of the apparatus by impurities, apparatus maintenance operations such as parts replacement and cleaning must be minimized. However, if the apparatus is continuously used for a long period of time, source materials are undesirably deposited on the parts of the apparatus. In particular, an ion gauge and a substrate heating device tend to receive the influences of source material deposition.
The molecular beam intensities are measured prior to epitaxy. If the apparatus has been used for a long period of time, the source materials are deposited on the collector and grid of the ion gauge and the protective walls surrounding the ion gauge. The absolute sensitivity of the ion gauge is changed with time due to the deposited materials. If a source material having a high vapor pressure is deposited, it is evaporated again by radiation heat from the filament. The background vacuum pressure is neither increased sufficiently nor stabilized, resulting in measurement variations and noises. Therefore, the molecular beam intensities can not be accurately measured.
Source material deposition is also harmful for the substrate heating device. First, since an element of a high vapor pressure among the source materials is evaporated again by radiation heat from the heating device during epitaxial growth of a thin semiconductor film, the resultant film composition is changed. Second, in order to improve uniformity of the grown film, the heating device has a rotary mechanism which rotates about its axis. Source material deposition interferes with smooth rotation of the rotary mechanism and causes a dust source during rotation. When dust is attached to the substrate, it serves as an abnormal growth nucleus to degrade the quality of the grown film or adversely affect the subsequent semiconductor device fabrication process.